T cell receptor (TCR) mediated signal transduction involves a complex series of early events that culminate in cellular activation. The zeta subunit of the TCR has several characteristics that make it unique. It is structurally unrelated to the other invariant receptor subunits; it is expressed in natural killer cells in which other receptor components are not expressed; it undergoes tyrosine phosphorylation in response to T cell receptor activation; and has been shown to transduce signals independently of other receptor components. We are interested in understanding the molecular mechanisms whereby receptor occupancy is coupled to intracellular events. our studies have concentrated on understanding the role that zeta plays both as an effector molecule and as a substrate for receptor activated protein tyrosine kinases. Studies are carried out primarily in the murine T lymphocyte hybridoma 2B4 and in variants of this hybridoma. Studies in 2B4 cells have been concentrated in two areas: systematic mutagenesis of the tyrosine residues of the zeta subunit; and development of a permeabilized cell system for manipulation of the intracellular environment. Studies in permeabilized cells have focused primarily on establishing whether GTP binding proteins play a role in TCR-mediated signalling.